2,6-DMN, which may be converted into naphthalene-2,6-dicarboxylic acid by oxidation, has attracted attention as an important material for the production of industrial products such as polyesters or plasticizers. 2,6-DMN is present in a petroleum fraction or a coal tar fraction in the form of a mixture with other DMN isomers. However it is difficult to economically obtain highly pure 2,6-DMN through distillation, since there are ten DMN isomers and 2,7-DMN, among all, has a boiling point close to that of 2,6-DMN. Additionally, it is known that DMN isomers form an eutectic mixture when crystallized. In particular, 2,6-DMN forms a two-component eutectic mixture together with 2,7-DMN or 2,3-DMN, which makes it difficult to recover 2,6-DMN at a high selectivity from a DMN mixture. Accordingly there have been proposed a number of methods for separating 2,6-DMN. For example, JP-B-47-29895 and JP-B-47-38440 disclose a method wherein 2,6-DMN is separated by forming a complex thereof together with m-nitrobenzoic acid, while JP B-55-44734 discloses a method wherein 2,6-DMN is separated by forming a complex thereof with p-nitrobenzoic acid. (The term "JP-B" as used herein means an "examined Japanese patent publication"). However each of these methods requires a complicated procedure in order to collect a composition mainly comprising 2,6-DMN by decomposing the formed complex with an alkali. Further JP-B-55-47021 proposes a method for separating 2,6-DMN by forming a complex thereof with nitrobenzene derivatives, but a satisfactory selectivity can not be achieved by this method.